add mid radius inf sup diam for reals

[lucaferranti]

fixes #510

[codecov-commenter]

Codecov Report

Merging #511 (ca59f94) into master (1e1dad2) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
+ Coverage   90.32%   90.35%   +0.02%     
==========================================
  Files          25       25              
  Lines        1788     1793       +5     
==========================================
+ Hits         1615     1620       +5     
  Misses        173      173              

Impacted Files	Coverage Δ
src/intervals/arithmetic.jl	97.67% <100.00%> (+0.04%)	⬆️

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[lbenet]

LGTM! Thanks a lot!